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Appendix  VI.                          1025

under all circumstances, mitre wheels r and ^ engage through the
medium of a face wheel /, and the motion is transmitted to the
tool spindle through the spur gear v. The interposition of the
face wheel permits therefore a connection of r and s, whatever
the angular relation of a and b.


P.   193.     Relieving   or   Backing-off  Lathe:—In  the

description of the making of screw-taps, p. 192, the clearance or
backing-off was said to be done by hand, but this is by no means
the best or proper method of doing it. - Taps are now relieved or
backed-off by the aid of some machine appliance, which gives the
turning or screwing tool such a transverse motion as will cause it
to cut deeper into the tap at certain times, as shewn in section at
Fig. 203, p. 194, the flutes having been previously cut as at A,
Fig. 185. Relieving lathes, specially built for backing-off while
turning, have a much wider and more important field of applica-
tion, however, in the making of milling cutters of every con-
ceivable form, and the machine shewn in Figs. 934, 934^, and
93 4^, built by Mr. J. E. Reinecker, and introduced in England by
Messrs. Pfeil & Co., from whom the drawings have been obtained,
will now be described.

The general view of the lathe is given in Fig. 934, the lower
diagram being a section through the bed, so as to shew the
arrangement of the shafts. The driving is as usual by cone
pulley A and back gear, and the milling cutter to be formed is
mounted on a bar and placed between the cone centres so as to
rotate at a suitable speed for the cutting. The cutter blank has
been previously roughed out in the manner shewn at ?n, where
the deep radial grooves indicate the number of teeth to be
adopted. These grooves are generally of spiral form, having long
pitch, so can easily be* done in the ordinary milling machirte
between centres that are slowly rotated by what is called a spiral
attachment. The object of the relieving lathe is next to give
each tooth of the cutter m its proper clearance angle by a process
of turning, the material being removed down to the dotted lines.